Selenium is a naturally occurring element that can occur in several oxidation states. It can exist in the [−II] (selenide), [O] (elemental selenium), [+IV] (selenite), and [+VI] (selenate) oxidation states. Selenium can be stabilized in aqueous solutions with CN− to form selenocyanate or by organic compounds to form mercaptan analogues such as dimethylselenide or isoselenocyanates. Selenium is a ubiquitous element having an average concentration of about 0.7 ppm in the earth's crust, and is concentrated in such diverse things as plants, sulfur deposits, sulfide minerals of copper and molybdenum, and fossil fuels. As a result, selenium can be found in waste streams from copper refining, acid coal mine drainage, coal-fired power plants, and petroleum refining. Selenium is generally considered to be hazardous, and selenium disposal is carefully regulated.
Selenium has also been found in waste agricultural irrigation water. In this regard, the presence of selenium in waste agriculture and irrigation water recently has become a major pollution concern. Selenium poisoning has been implicated in waterfowl deaths and deformities at the Kesterson National Wildlife, Refuge near Los Banos, Calif. The potential of selenium poisoning, therefore, has generated significant interest in a number of different industries where research activity has been directed to the removal of selenium from wastewater.
A particularly acute problem of concentrated selenium discharge occurs in the waste waters from petroleum refineries. Many refineries have this problem, to a lesser or greater extent depending on the origin of the crude oil. As the selenium is isomorphous with sulfur, it accompanies sulfur in the processing of the oil. If selenium is present it generally accompanies sulfur in the oil. Crude oil taken from the San Joaquin Valley in Calif., for example, has relatively high selenium content. Most of the sulfur and selenium found in crude ends up in refinery sour water streams which are subsequently treated by sour water strippers. However, while the stripping of hydrogen sulfide from sour water in conventional sour water strippers is highly efficient, significant amount of selenium compounds remains in the stripped sour water. The predominant selenium compounds remaining in the stripped sour water are hydrogen selenide and selenocyanate. Minor amounts of elemental selenium and oxidized forms such as selenite and selenate might also be present. Typically the stripped sour water, containing selenium compounds, is directed for further treatment with the rest of the refinery wastewater and in the process it gets oxidized to elemental selenium, selenite and selenates. As a result, in the waste water discharge from refineries processing crude oil high in selenium, selenium content has been high. In the case of several refineries located in the San Francisco Bay area the concentration of selenium in the wastewater could be above accepted levels for safe discharge.
As noted above, selenium is introduced to the refinery process through the crude oil which is processed in the refinery. It may be partially removed in the initial crude dewatering and desalting steps, but much of the selenium is carried over into petroleum processing, such as hydroprocessing or fluid catalytic cracking. Selenium present in the products from these processes is removed by aqueous treatments such as washing and absorption. The aqueous streams from the various aqueous treatment processes in the refinery are generally combined into a sour water stream, which is treated for recycle and/or for disposal.
Sour water is process water recovered from petroleum or hydrocarbon streams during refinery operations. For example, sour water may be recovered from the petroleum streams, as in crude oil dewatering, it may be recovered from a washing process, such as during crude oil desalting, or it may be recovered from an absorption process, such as, for example, removing sulfur and nitrogen compounds from hydroprocessed products. Sour water generally contains soluble oil and free oil contaminants, inorganic ions such as ammonia, sodium, sulfates, sulfites, chlorides, CO2, OH and also solids that are typically corrosion products such as iron sulfides, iron oxides or hydrates etc.
Sour water is typically processed in a sour gas stripper. A sour gas stripper is a single or multi-stage separation zone for treating sour water. The stripping action may be facilitated by the introduction of a hot gaseous stripping medium, such as steam. The overhead stream from the sour gas stripper may include ammonia, hydrogen sulfide, purified water vapor, or combinations thereof, depending on the particular process. The bottoms product from the stripper is a stripped sour water stream. The stripped sour water stream generally contains the majority of the selenium compounds. Efforts to remove the selenium from a sour water stream were unsuccessful so far. Effluents of sour water strippers are difficult to treat for selenium removal because of the unpredictable nature and unpredictable quantities of contaminants that are present in the effluents. These contaminants often hinder irreversibly selenium removal processes that use membrane, ion exchange resins or inorganic adsorbents. The method of the present invention is especially useful for removing selenium from sour water in the presence of contaminants of variable composition and quantity, especially in the presence of free and soluble oil. However, any aqueous stream may be usefully treated using the present method.
Because the selenium concentration in a typical refinery wastewater effluent is generally very low, i.e., typically in the ppb range, wastewater treatment techniques that have been used to remove selenium have been of the adsorption type such as iron and aluminum hydroxide adsorption. Also, reverse osmosis techniques have been attempted and were found to be very costly and difficult to operate due to membrane fouling with free oil and solids. The success of the various adsorption methods depends largely on the selenium species present and on competitive ions in solution.
As noted above, selenium can exist as selenide, elemental selenium, selenite, selenate, and selenium complexes with cyanite or organic bases. Of these species, ion exchange favors selenocyanate over selenate and selenate over selenite, whereas the iron hydroxide adsorption has no affinity for selenocyanate and favors selenite over selenate. Since most refinery final effluents and natural waters include a mixture of selenate and selenite selenium species, it has been difficult to approach complete removal of selenium from refinery effluents or natural water using only one step. Furthermore, oxidation to, or reduction from, the selenate state is kinetically very slow which further inhibits optimization. Ion exchange also has not been a successful removal technique because selenate shows almost identical resin affinity as sulfate, which is usually present in a concentration of several orders of magnitude higher than selenate. Thus, the sulfate simply preferentially competes with selenium for resin sites. Furthermore, ion exchange resins become fouled when used to treat selenium wastewater and methods for regeneration are often inadequate and unpredictable.
Among other things, the method of the present invention quantitatively, and inexpensively, removes selenium from stripped sour water prior to combining the stripped sour water with other refinery wastewater for further processing.
As noted previously, the method of the present invention is especially effective with respect to the removal of selenium from the strip sour water effluent waste waters produced from petroleum refineries, although it is useful with other industrial effluent waters as well. Industrial processes include synfuel from coal conversion and many metallurgical processes, particularly precious metals process solutions that involve cyanidation of selenium containing precious metal ores or concentrates. The latter includes mineral processing waste, and particularly heap leach gold mine washings. In the case of some industrial processes there will also be, or there will alternatively be, molybdenum and/or other toxic metals present. The method of the invention is effective to remove most such toxic metals, as well as selenium.
Current methods for removing selenium from stripped sour water involve ion exchange resin. In these methods, the selenium-containing sour water passes over an ion exchange resin, which selectively adsorbs the selenium. While this method is effective, ion exchange resins are very expensive. Further, their use in this service is severely limited by the tenacity with which selenium, particularly selenocyanate, irreversibly adsorbs on the ion exchange resin. Methods for removing the selenium and regenerating the resin are generally ineffective, thus rendering the resin permanently deactivated. The present invention provides, among other things, a way to quantitatively remove the selenium from stripped sour water in a cost effective manner by passing a selenium-containing aqueous stream in combination with a quaternary amine compound through a filter and recovering a treated aqueous stream which is depleted in selenium content relative to the untreated selenium-containing aqueous stream.
An example of an ion exchange process is disclosed in U.S. Pat. No. 4,915,928 to Marcantonio. The patent relates to a process for removing selenium from wastewater effluent which includes the steps of; (i) contacting a selenide containing wastewater and a strong-base anion exchange resin to absorb selenide on the resin; (ii) eluting the ion exchange resin with an eluant which is effective for stripping selenide therefrom; and (iii) recovering elemental selenium from the selenide containing eluate resulting from step (ii).
A process for removing selenium, present in the waters as selenocyanate, by precipitation with copper (II) salts was shown to effectively precipitate selenium under laboratory conditions. In this process the selenium is precipitated as an alpha Cu(S0.91Se0.09)CN solid solution. However, this process, although successful under controlled conditions did not work well in an actual refinery operation.
There is a need for a reliable process for removing selenium from refinery wastewaters under actual refinery operating conditions where the amount of free oil contaminants and soluble oil contaminants vary unpredictably. The present invention, among other factors, provides such a process.